Network Working Group W. Simpson
Request for Comments: 1619 Daydreamer
Category: Standards Track May 1994
PPP over SONET/SDH
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
The Point-to-Point Protocol (PPP) [1] provides a standard method for
transporting multi-protocol datagrams over point-to-point links.
This document describes the use of PPP over Synchronous Optical
Network (SONET) and Synchronous Digital Heirarchy (SDH) circuits.
This document is the product of the Point-to-Point Protocol Working
Group of the Internet Engineering Task Force (IETF). Comments should
be submitted to the ietf-ppp@merit.edu mailing list.
Applicability
This specification is intended for those implementations which desire
to use the PPP encapsulation over high speed private point-to-point
links, such as intra-campus single-mode fiber which may already be
installed and unused. Because the PPP encapsulation has relatively
low overhead, it is anticipated that significantly higher throughput
can be attained compared to other SONET/SDH payload mappings, at a
significantly lower cost for line termination equipment.
Simpson [Page i]

RFC 1619 PPP over SONET/SDH May 19941. Introduction
PPP was designed as a standard method of communicating over point-
to-point links. Initial deployment has been over short local lines,
leased lines, and plain-old-telephone-service (POTS) using modems.
As new packet services and higher speed lines are introduced, PPP is
easily deployed in these environments as well.
This specification is primarily concerned with the use of the PPP
encapsulation over SONET/SDH links. Since SONET/SDH is by definition
a point-to-point circuit, PPP is well suited to use over these links.
The Synchronous Optical Network (SONET) [3] is an octet-synchronous
multiplex scheme that defines a family of standard rates and formats.
Despite the name, it is not limited to optical links. Electrical
specifications have been defined for single-mode fiber, multi-mode
fiber, and CATV 75 ohm coaxial cable. The transmission rates are
integral multiples of 51.840 Mbps, which may be used to carry T3/E3
bit-synchronous signals. The allowed multiples are currently
specified as
STS-1 51.840 STS-18 933.120
STS-3 155.520 STS-24 1,244.160
STS-9 466.560 STS-36 1,866.240
STS-12 622.080 STS-48 2,488.320
The CCITT Synchronous Digital Heirarchy (SDH) defines a subset of
SONET transmission rates beginning at 155.520 Mbps [5].
SONET SDH equivalent
STS-3c STM-1
STS-12c STM-4
STS-48c STM-16
2. Physical Layer Requirements
PPP treats SONET/SDH transport as octet oriented synchronous links.
SONET/SDH links are full-duplex by definition.
Interface Format
PPP presents an octet interface to the physical layer. There is
no provision for sub-octets to be supplied or accepted.
Simpson [Page 1]

RFC 1619 PPP over SONET/SDH May 1994
The octet stream is mapped into the SONET/SDH Synchronous Payload
Envelope (SPE), with the octet boundaries aligned with the SPE
octet boundaries.
No scrambling is needed during insertion into the SPE.
The Path Signal Label (C2) is intended to indicate the contents of
the SPE. The experimental value of 207 (cf hex) is used to
indicate PPP.
The Multiframe Indicator (H4) is currently unused, and MUST be
zero.
Transmission Rate
The basic rate for PPP over SONET/SDH is that of STS-3c/STM-1 at
155.520 Mbps. The available information bandwidth is 149.760
Mbps, which is the STS-3c/STM-1 SPE with section, line and path
overhead removed. This is the same super-rate mapping that is
used for ATM and FDDI [4].
Lower signal rates MUST use the Virtual Tributary (VT) mechanism
of SONET/SDH. This maps existing signals up to T3/E3 rates
asynchronously into the SPE, or uses available clocks for bit-
synchronous and byte-synchronous mapping.
Higher signal rates SHOULD conform to the SDH STM series, rather
than the SONET STS series, as equipment becomes available. The
STM series progresses in powers of 4 (instead of 3), and employs
fewer steps, which is likely to simplify multiplexing and
integration.
Control Signals
PPP does not require the use of control signals. When available,
using such signals can allow greater functionality and
performance. Implications are discussed in [2].
3. Framing
The framing for octet-synchronous links is described in "PPP in HDLC
Framing" [2].
The PPP frames are located by row within the SPE payload. Because
frames are variable in length, the frames are allowed to cross SPE
boundaries.
Simpson [Page 2]